This invention relates to the treatment of coal ash, and particularly to a method for reburning bottom ash and flyash components of coal to remove carbon and provide a usable end product.
Coal fired boilers are widely used to generate steam for producing electricity. A common form of boiler uses a pulverized coal that is injected into a furnace. Millions of tons of coal ash result each year from such operations. The coal ash includes flyash with a minor proportion of bottom ash. Some of the ash is commercially usable in concrete, concrete products, cement production, sewage sludge stabilization, pavement base materials, lightweight aggregate, and other miscellaneous purposes. The remaining coal ash must generally be disposed of by landfilling since it has no commercial value. A principal reason for a lack of commercial value for coal ash is the presence of unburned carbon in the ash. According to ASTM Standard C618, an ash must have a Loss On Ignition (LOI) value no higher than 6% for use in concrete. An upper limit of 3% is more realistic. Higher LOI ash cannot be used because of color problems and concerns for durability under freezing and thawing conditions.
The residual carbon content in the coal ash depends upon a variety of factors including base line furnace operation and boiler design. It also depends upon the source of the coal fuel. For example, subbituminous western coal, when burned, will typically result in a low carbon ash with an LOI of less than 5%.
A variety of methods for dealing with the high carbon flyash problem are currently under investigation. The methods include froth flotation which uses mining technology for separation of materials of different densities, electrostatically-charged belts to separate carbon from the flyash, a fluidized bed separation using acoustical techniques, and combustion of the remaining carbon in the flyash in a fluidized bed boiler designed specifically for that purpose. None of these technologies have demonstrated the ability to completely remove the LOI on a commercial scale, and all of these approaches would require significant capital and operating costs.
It is a principal object of the invention to provide a method of treating high carbon coal ash using existing capital installations, and particularly existing pulverized coal boilers.